Mg/Ca Ratios in Stressed Foraminifera, Amphistegina gibbosa, from the Florida Keys

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calcium, foraminifera, inductively-coupled plasma mass spectrometer (icp-ms), magnesium

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Since 1991, significant proportions of Amphistegina populations in the Florida Keys and elsewhere have exhibited stress symptoms that include loss of symbiont color (‘bleaching’), anomalous shell breakage and reproductive damage. Previous studies of other taxa have reported elevated Mg/Ca ratios in tests from pollution-stressed foraminiferal populations. The purpose of this study was to test the hypothesis that anomalous shell breakage in stressed Amphistegina gibbosa is the result of loss of control of calcification, resulting in elevated concentrations of Mg that weaken the crystal structure of the test.

Analysis of Mg and Ca concentrations in A. gibbosa tests, using an Inductively-Coupled Plasma Mass Spectrometer, revealed normal Mg/Ca (2–5 mol%) in all specimens analyzed, including normal specimens collected in 1982 (prior to the onset of the stress event), and both normal and broken specimens collected quarterly from afflicted populations in 1996. Analysis of specimens from the high Mg calcite taxon, Archaias angulatus, revealed Mg/Ca of 10–14 mol%. This study, which presents an ICP-MS procedure that can be used to assess Mg/Ca in individual foraminifera, does not support the hypothesis that shell breakage in stressed Amphistegina results from disruption of calcification at the ionic level.

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Citation / Publisher Attribution

Marine Micropaleontology, v. 43, issue 3-4, p. 199-206